In order to maximize the network capacity and mobile terminal battery life, cellular telephones must accurately control the transmitted radio frequency (RF) power over a dynamic range of 30 dB or more. This is accomplished by controlling the power amplifier (PA) in the cellular telephone using a ‘closed loop’ method to continually adjust the gain control of the PA based on an integration of the error signal between a reference current, and a power detector (PD) output. A logarithmic power detector (LPD) provides a current or voltage output that is a linear function of the input power. The operation of a LPD is based on a gain curve that is an approximation to the logarithm function. This ‘logarithmic’ approximation is generally accomplished through the amplification of an input voltage signal by a chain of limiting amplifiers. The output of each gain stage is then converted into a current, rectified, and summed with the rectified outputs of the remaining stages. This forms the ‘demodulated’ output, in that the DC output current of the LPD is proportional to the logarithm of the signal envelope. This technique of generating the logarithmic gain function is known as ‘successive compression’ or ‘progressive-compression’. Progressive-compression type logarithmic amplifiers synthesize a logarithmic function through progressive compression of the input signal over many amplifier stages. Each amplifier stage has a relatively low linear gain (typically two to four) up to some critical level. Above the critical level the incremental gain of the amplifier stage is reduced, and in some cases is zero. In demodulating logarithmic amplifiers, the input signal is typically an RF signal, and the output is a signal that is proportional to the logarithm of the input signal envelope.
Referring now to FIG. 1, prior art demodulating logarithmic amplifiers typically include a number of serially coupled amplifier stages 14, a number of linear transconductance or voltage-to-current converter elements 16, and a number of full-wave rectifier cells 15. The transconductance stages are coupled to the output of each amplifier, as well as an initial, transconductance stage 16a coupled to the input of the first amplifier 14a. The output from each transconductance stage is input into a full-wave rectifier, and the output current from each rectifier is summed on a current bus 18. This summed current is then converted from a current into a voltage by a resistor at the output 19.
The challenge in utilizing the demodulating logarithmic amplifier in FIG. 1 as a logarithmic power detector in a cellular handset is that it must operate predictably over a wide range of power amplifier (PA) carrier frequencies and output powers (≧50 dB dynamic range). For example, in a tri-mode GSM mobile station, the PA module must operate at carrier frequencies in the vicinities of 900 MHz (GSM), 1.8 GHz (DCS), and 1.9 GHz (PCS). Over this wide range of frequencies and output powers, the power control loop (and therefore the LPD) must track the envelope fluctuations of the PA output so as to maintain an absolute power accuracy dictated by the appropriate governing standard. The shortcoming for the LPD in FIG. 1 is that in order to track the envelope fluctuations (and therefore the power) of the PA output signal, each of the components in FIG. 1 must be designed to operate at the appropriate RF carrier frequency. If the LPD in FIG. 1 is to be used for all three bands, it becomes necessary to maintain the required voltage gain within each amplifier cell over an extremely wide bandwidth. In addition, the voltage-to-current converters 16 and the rectifiers 15 in FIG. 1 must also be very fast. These high gain-bandwidth components can be so stringent that the device must be fabricated using very expensive semiconductor processes. A need exists for a demodulating logarithmic amplifier that is less complex, uses fewer components, and that can be manufactured at a lower cost.